The present invention relates to a circuit for operating a D.C. circuit breaker and, more particularly, to a circuit for operating a D.C. circuit breaker for breaking when the instantaneous value of current flow to the circuit breaker is zero.
FIG. 1 is a schematic view of the construction of a prior art D.C. circuit breaker disclosed, for example, in Japanese Patent Publication No. 41-12533. In FIG. 1, numeral 1 designates a D.C. power source connected to a series circuit of a load 2 and a circuit breaker 3. Further, a series circuit of a switch 4, an inductor 5, and a power source 6 including a capacitor previously charged by a charging device (not shown), hereinafter referred to as "a capacitor power source", is connected in parallel with the circuit breaker 3.
FIG. 2 exemplifies a current waveform diagram of a current flowing to the circuit breaker 3 in a prior art example of FIG. 1 when a breaking operation is executed.
Assume now that the switch 4 is closed at time t.sub.1 and that the circuit breaker 3 is to be opened simultaneously. Thus, a current flowing to the circuit breaker 3 changes to become zero at a time t.sub.2 or at a time t.sub.2 ', depending upon the polarity of the capacitor power source. It is desirable to open the contacts of the circuit breaker 3 only when the current flowing thereto is substantially small and preferably zero (at either t.sub.2 or t.sub.2 '). However, when the circuit breaker 3 fails to open at the time t.sub.2 or t.sub.2 ', the initial interrupting operation of the circuit breaker 3 is repeated in the next cycle so as to open the contacts when the current flowing to the circuit creaker 3 becomes zero. The inductor 5 is provided to regulate a time constant for defining a period from the initial zero crossing point of the current flowing to the circit breaker 3 to the zero crossing point of the next cycle.
However, with this prior art D.C. circuit breaker arrangement, the current flowing to the D.C. circuit breaker quickly reduces to zero when a capacitor power source with reverse polarity (as compared to the D.C. power source) is used. Also, the time difference (t.sub.2 -t.sub.1) is short and insufficient to open the contacts due to physical and mechanical limitations of the circuit breaker. As a result, the contacts are often opened when the current flowing to the circuit breaker is not zero, thus generating an arc plasma between the electrodes (contacts) of the circuit breaker. Therefore, the operating life of the circuit breaker is drastically shortened and frequent replacement is required. Furthermore, there arises a problem that a reverse current flowing in the reverse direction at the next cycle makes it very difficult to determine the actual zero crossing point and, hence, the interruption operation of the circuit breaker becomes ineffective.